1. Field of the Invention
The present invention relates to an apparatus and method for evaluating the optical properties of a semiconductor light emitting diode (LED), and more particularly, to an apparatus and method for evaluating the optical properties of an LED and a method for manufacturing an LED device.
2. Description of the Related Art
A white LED device, which is currently in the spotlight as a lighting device, is manufactured by combining a blue LED chip or an ultraviolet (UV) LED chip with phosphors which convert the wavelength of light emitted from the LED chip to generate visible light. In order to obtain a target Commission Internationale de l'Eclairage (CIE) chromaticity and a desired optical output when manufacturing such a white LED device, the optical properties of the LED chip and the optical properties of the phosphors should be combined to accurately set output properties including a target CIE chromaticity, a dominant wavelength, an optical output, and a light speed.
To manufacture white LED devices belonging to the same white CIE chromaticity group, the optical properties of LED chips are measured, and the LED chips of which the optical properties are measured to be the same rank depending on a predetermined standard are classified into the same group. The LED chips classified into the same group are mounted on packages, respectively, and a proper amount (or mixing ratio) of phosphor is dispensed around the LED chips to manufacture the white LED devices. Typically, transparent resin including the phosphor is dispensed. For the LED chips classified into another group, a different amount (or mixing ratio) of phosphor may be dispensed. Then, the optical properties of the manufactured white LED devices are measured, and the white LED devices which satisfy target optical efficiency and a target white CIE chromaticity are classified and shipped.
In the above-described processes, the process of measuring the optical properties of the LED chips is referred to as a probing process. Depending on how the optical properties of the LED chips are measured by the probing process and how the LED chips are classified depending on the measurement result, the production yield of the white LED devices may be decided. In general, the optical properties of LED chips having an effect upon the optical properties of white LED devices include a wavelength and an optical output. When the LED chips are classified depending on the optical properties, a dominant wavelength or peak wavelength may be used as the wavelength, and the optical output may be used in unit of mV or mcd. When the LED chips are classified depending on the wavelength and the optical output, the LED chips may be classified in such a manner as to have the correlation with the optical properties of white LED devices which are to be manufactured. However, it is difficult to accurately measure variations in wavelength or optical output depending on viewing angles. Furthermore, when the optical properties of the LED chips are measured, a short wavelength of light such as blue light or UV light is measured. Therefore, the variation of the optical properties is very small. Accordingly, it is very difficult to measure the optical properties of the LED chips such that the optical properties have the correlation with the optical properties of the white LED devices. Hence, although LED chips classified into the same group are used to manufacture white LED devices through the same package process, the white LED devices may exhibit different chromaticities and optical outputs, and some of them may exhibit chromaticities deviating from the target chromaticity range.
An apparatus for measuring the optical properties of an LED according to the related art receives monochromatic light such as blue light or UV light emitted from LED chip and measures the light quantities and wavelengths of the monochromatic light. Depending on the measured light quantities and wavelengths, LED chips having a constant optical property are grouped and classified. However, despite the variations in wavelength, the movement of the chromaticities of blue or UV LED chips used in the white LED devices is much smaller than that of the chromaticities of the white LED devices. Accordingly, although the blue or UV LED chips are classified into the same group depending on the light quantities and wavelengths, the chromaticities of the white LED devices implemented by dispensing phosphor to the LED chips have considerably wide distribution. Light quantity (luminous intensity or optical output) has a close relationship with chromaticity. For example, when a white LED device is implemented by using a blue LED chip and yellow phosphor, the chromaticity of the white LED device differs depending on the ratio of the quantity of blue light to the quantity of yellow light which is obtained from the phosphor by the blue light. Therefore, when a slight difference occurs in the light quantity of the blue light, the chromaticity of the white LED device is varied, and the white light is affected.
Furthermore, due to a deviation between elements of the white LED devices, including an LED chip, a package body, a lead frame, a phosphor, and a sealing agent, a target chromaticity may be not obtained. Therefore, it is difficult to finely control the chromaticity of the white LED device to the target chromaticity by classifying the LED chips through the chip probing. The chromaticity of the white LED device may be affected by the shape of a lead frame, the position of the LED chip inside the package, and the amount of resin sealing agent to be dispensed. However, main factors of the chromaticity distribution of the white LED device may not be discriminated. Accordingly, the cause of the chromaticity distribution may not be properly investigated, and there is a considerable obstacle to improving the production yield of the white LED devices in terms of the chromaticity.
To reduce the chromaticity distribution of white chromaticities, a small number of LED chips may be sampled before phosphor-containing resin is dispensed, that is, before the dispensing process. Then, the dispensing process may be performed on the sampled LED chips to finely control the chromaticity thereof. In this case, since the entire process (curing and so on) after the dispensing process is performed until the chromaticities of the sampled chips reach the target chromaticity, there is a considerable amount of cost and time consumption involved. Furthermore, the target chromaticity is not always guaranteed for the LED chips other than the sampled LED chips.
The combination of monochromatic light emitted from the LED chip and light emitted from the phosphor may generate a specific color of light other than white light. Even when an LED device outputting a specific color of light other than white light is manufactured, it is necessary to realize the target chromaticity, to reduce the chromaticity distribution, and to increase the yield.